GetOrbitPosition places a parent as a side effect of placing its child, so with a child listed first the parent's previous position was already its new one and its Velocity came out zero. The constructor's unread PreviousPosition write is dropped; the first Update takes it from the placed Position. The velocity test now runs moon-first and planet-first packs over varying dt. Builds not verified yet (verify follows).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>